Not Applicable
Not applicable
This invention relates to magnetic resonance imaging coils, specifically to such coils that are arrays comprising of multiple smaller coils.
In a typical magnetic resonance imaging (MRI) examination activity, an imaging coil is placed over the anatomy of interest and then the patient is placed inside a large magnet. The static field of the magnet aligns the protons in the patient""s body in the direction of its magnetic field. A large whole body coil is used to generate high-energy (peak power 5-15KW) radio frequency (RF) pulses that excite protons in the patient. These protons relax back to their original state after the excitation is switched off At this time the scanner automatically switches xe2x80x98ONxe2x80x99 the imaging coil and turns the body coil xe2x80x98OFFxe2x80x99. As the protons relax they induce minute RF signals in the imaging coils. These signals are picked up by the imaging coil and fed to a signal processing system from where the information is transferred to a computer for image reconstruction and display.
Signal to noise ratio (SNR) and field of view (FOV) are the two most important parameters for characterizing a MRI imaging coil. SNR is a measure of the sensitivity of the imaging coil and its intrinsic electric resistance. Higher the sensitivity of a coil greater is the signal voltage induced in the coil. A coil with higher intrinsic resistance generates more noise than the one with lower intrinsic resistance. FOV is a measure of the region over which the coil is sensitive enough to pick-up acceptable SNR signals. The ultimate MRI imaging coil can be defined as the one with the largest FOV and the highest SNR over that FOV.
Single loop imaging coil is the most basic MRI coil. For a single loop coil there is the diameter of the loop determines the SNR and the FOV of the coil. Larger the diameter of the coil larger is its FOV. However large diameter coils have higher intrinsic resistance because the induced currents have to travel over a longer conductor path along the circumference in comparison to a smaller diameter loop coil. Therefore, in the past, designers have had to optimize the diameter of such loop coils depending on the application and make a choice between SNR and FOV.
U.S. Pat. No. 4,825,162 describes a method for simultaneously receiving NMR signals from a plurality of closely-spaced imaging coils. This technique overcomes the tradeoff between SNR and FOV by using multiple small sized coils for simultaneous data acquisition. Such coils are commonly referred to as phased array coils or coil arrays. Depending on the anatomy several designs exist for these coil arrays. The most well known coil array is the four element spine array. As the name suggests this is made of four identical circular loops placed side by side in a line with adjacent loops slightly overlapping each other. Signals from each of these loops is acquired simultaneously and digitally recombined, such that a long FOV is generated. Such a FOV can be generated using a single loop coil with a much larger diameter. However the SNR would suffer as the large loop has lower sensitivity than each of the smaller loop coils. Thus, by recombining signals from an array of smaller coils it is possible to get high SNR as well as a large FOV.
When MRI coils that are tuned to the same radio frequency are brought together in close proximity as in phased array coils, they inductively couple to each other and detune each other. This results in image quality that is worse than what can be achieved when each coil is used by itself. Phase array coil technology overcomes this problem by slightly overlapping adjacent coils and by using specially designed preamplifiers that dampen the current flow in the individual coils.
In order to get the best image quality coil arrays are placed as close as possible to the anatomy under examination. The array housing is specially contoured to fit a particular part of the body such that the anatomy of interest lies well within the area of highest sensitivity of individual coil elements. Since the FOV needed for every application is different, the number and the type of elements used in a array coil vary from one array to the other. This leads to an MRI clinic investing in a unique array for every application, for example, a special spine array with a long FOV is used for spine imaging while a special knee array with much smaller FOV is used for knee imaging. A typical spine array has six imaging coil elements while a knee coil has only four elements.
During an MRI scan a patient can get burnt if the coaxial cable of the imaging coil loops over itself Large RF current can get induced in the loop from the RF energy transmitted by the body coil. Also, the cable it self can act like an antenna and have RF standing waves form on the outer shield of the coaxial line. All these can result in RF burns to the patient. These issues have been overcome by mechanically making cables less prone to looping, by using Baluns and RF traps to prevent any RF currents from flowing on the cable shield.
Cable burn issues are an even greater concern with phased arrays since there are as many cables as there are imaging coil elements. Care is taken to anchor the cable in special jackets to prevent them from looping. Additionally the coils them selves are anchored in preset location inside coil housing to minimize their interaction with each other and their cables. Individual coil overlap is preset during manufacture of the coil array and cannot be changed. Also, since all the elements of the array are together if one element or the cable assembly fails then the whole array is sent back to the factory for repair.
Phased array coils such as those described in U.S. Pat. No.: 6,084,411 and 5,905,378 restrict a user to coil arrangements in which adjacent coils elements have to be in a fixed predetermined overlap position in order to minimize inter-coil coupling locations and minimize the loss in SNR due to coupling. Coils that are not overlapped have to be physically separated at a minimal distance such that they do not couple to each other. This limits the configurations the device can be used in. For example, these coils cannot be used in a geometrically opposed manner in which two coils face each other without significantly degrading SNR. Further, the coils are isolated from the cables through tuned Baluns. Another drawback of using this approach is that the tuned Balun introduces an insertion loss that adds to the overall noise in the system. Also, this approach does not eliminate bums that can occur from a cable looping over it-self
All the phased array coil heretofore known suffer from a number of disadvantages:
(1) they are all application specific coils with dedicated coil elements,
(2) adjacent coil elements have to be overlapped in order to minimize interactions with each other,
(3) coil elements are not removable or exchangeable between array configurations,
(4) users have limited options for optimizing array configurations to match their needs and
(5) if an individual coil element fails then the whole array assembly has to be sent for repairs.
The forgoing illustrates limitations known to exist in present phased array coils. It is apparent that it would be advantageous to provide an improved phased array coil, such as an array coil with separable imaging coil elements, and which is directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
The present invention creates a paradigm shift in the current art of MRI imaging coil arrays. In one aspect of the present invention, a user can assemble different array coils from a set of basic building blocksxe2x80x94imaging coil elements and cable holders. These imaging coil elements can be used over and over to form array coils of various shapes and sizes. Moreover, since these coil elements are not dependent on overlap for minimizing interaction with other coils, the users have considerable freedom in placing coil elements to better match each individual application. The ability to move the elements relative to each other allows the MRI technicians to determine an optimal position to get the best image quality. These coil elements can be configured in various planar as well as three-dimensional arrangements to form arrays that are optimized for SNR and FOV.
In one embodiment, individual coil elements can be directly connected via a quick disconnect connector to a pre-formed cable holder to form a coil array. The same coil elements can be reused over and again on a different cable holder to form a different array.
It is another purpose of this invention to improve patient safety and minimize the possibility of cable burns by restricting the movement of cables used for connecting individual coil elements to the MRI scanner.
It is another purpose of this invention to allow a user to make a cost effective investment in a few standard sized coil elements that can be reconfigured to form new coil arrays versus investing in several dedicated application specific arrays.
It is another objective of this invention to produce modular arrays that can be easily field serviced and maintained. Defective modules that are easy to identify and can be exchanged in the field instead of the entire array being sent back to the factory for repair.